This invention relates to a method of making permanent magnets and more particularly to a method of making isotropic permanent magnets of manganese-aluminum-carbon (Mn-Al-C) alloys.
Previously known Mn-Al alloy magnets consisting of Mn 60.about.75 weight % (hereinafter referred to simply as %) and the remainder aluminum are such that the metastable phase (face-centered tetragonal, lattice constant a=3.94A, c=3.58A, c/a=0.908 and a Curie point of 350.degree. to 400.degree.C; hereinafter referred to as the .tau. phase) is obtained by way of a heat treatment such as by the cooling-control method or the quenching-tempering method. The .tau. phase is the metastable phase which appears between the high temperature phase (non-magnetic hexagonal structure, lattice constant a=2.69A, c=4.38A, c/a=1.63; hereinafter referred to as the .epsilon. phase) and the room-temperature phase in which the alloy is separated into the AlMn(.gamma.) phase (hereinafter referred to as .gamma. phase) and the .beta. -Mn phase (hereinafter referred to as .beta. phase). This intermediate phase was discovered by Nagasaki, Kono, and Hirone in 1955. (Digest of the Tenth Annual Conference of the Physical Society of Japan, Vol. 3, 162, October, 1955).
However, the above Mn-Al alloys possess magnetic characteristics which are low, i.e., in the order of (BH)max = 0.5 .times. 10.sup.6 G. Oe, Br = 2200 G, and .sub.B H.sub.C = 600 Oe. Since then, a method has been developed of sintering the powdered alloy in the .tau. phase whereby the coercive force is increased by pulverizing; however, the magnetic characteristics of these alloys, at best, were low, being in the order of (BH)max = 0.6 .times. 10.sup.6 G. Oe, Br = 1700 G, and .sub.B H.sub.C = 1250 Oe. Moreover, since the products were formed from powder, their mechanical strengths were low, which make these products impractical for commercial use.
On the other hand, in the attempt to improve the magnetic characteristics of those Mn-Al alloy magnets by making them anisotropic, a method of subjecting the alloy in .tau. phase (ferromagnetic phase) to high degree of cold working (hereinafter referred to simply as cold working method), a method of forming in a magnetic field the .tau. phase pulverized in cold into powder or the same method followed by sintering (hereinafter referred to simply as in-field powder forming method), etc., were proposed. Because of the difficulties to be described below, these anisotropizing methods have not yet found practical application.
Thus, in the cold working method of anisotropization, because Mn-Al alloy magnets are intermetallic compounds having hard and brittle mechanical properties, even a cold-working of less than 1% causes cracks or fractures in the alloys.
On the other hand, since the degree of anisotropization is dependent upon the degree of cold-working, it is necessary to cold-work the alloy to a high degree, normally higher than 80%, in order to achieve satisfactory magnetic characteristics. For this reason, the Mn-Al alloy will be broken and pulverized in the step of this working.
In order to overcome the difficulties, it is known that rod shaped Mn-Al alloy magnets in the .tau. phase are sealed in nonmagnetic stainless steel pipes such as 25Cr-20Ni stainless steel pipe, and while being held in said pipes are subjected to cold working, such as swaging, to a degree of 85.about.95%. This method is capable of producing an anisotropic permanent magnet possessing magnetic characteristics in the order of Br = 4280 G, .sub.B H.sub.C = 2700Oe, and (BH)max .apprxeq. 3.5 .times. 10.sup.6 G, Oe in the direction of preferred magnetization i.e., the axial direction of the rod. When this method is used, however, due to the intense working, the Mn-Al alloy magnets inside the pipe must be finely pulverized into powder, therefore only a slender bar sealed in stainless steel pipe can be obtained, and moreover, it is difficult to obtain rods of uniform cross-section. The method is therefore costly and of little practical value.
Furthermore, in order to overcome these difficulties, a method has been proposed of obtaining a rod shaped anisotropic Mn-Al alloy magnet by subjecting the .tau. phase of the Mn-Al alloy magnet to hydrostatic extrusion at a temperature below 200.degree.C, but the magnetic characteristics of such alloys is low, being in the order of (BH)max = 2.5.about.3.6 .times. 10.sup.6 G. Oe in the direction of preferred magnetization. This method also requires very intricate hydrostatic extrusion operation and is again a very impractical method.
On the other hand, the in-field powder forming method is a method whereby after pulverizing the .tau. phase (magnetic phase) of the magnet of Mn-Al alloy, the powder is press-formed in a magnetic field. In this instance also, even the best of the values of magnetic characteristics in the preferred direction of magnetization, was of the order of (BH).sub.max .apprxeq. 1.85 .times. 10.sup.6 G. Oe. Moreover, a difficult pulverizing process was required, and the permanent magnet obtained, being a product formed from powder, had the poor mechanical property of being very brittle, detracting from its practical use.
When the hot working, etc., is conducted, Mn-Al alloy has low stability at high temperature of the .tau. phase, being the magnetic phase; thus, above 530.degree.C, it undergoes a transformation into the .gamma. phase or .beta. phase in a short period of time, and at a temperature exceeding 830.degree.C, the transformation proceeds to the .epsilon. phase.
Furthermore, the transformation from the .tau. phase to the .beta. phase is accelerated by the working; thus the so-called strain induced transformation is brought, and as a consequence, all Mn-Al alloys which had been worked at a temperature above 530.degree.C showed markedly lowered magnetic properties, or became nonmagnetic.
The plasticity of Mn-Al alloys above mentioned was found to be based on the high deformability of the .beta.-phase, and not on the deformability of the .tau. phase itself.
As described hereinabove, in the Mn-Al alloys, above 530.degree.C, the transformation to the .beta. phase is induced, as the degree of working is increased, to the detriment of its magnetic characteristics.
To replace the Mn-Al alloy magnets mentioned above, there have been invented manganese-aluminum-carbon alloy magnets in bulk shape having excellent magnetically isotropic characteristics, which magnets were disclosed in U.S. Pat. No. 3661567.
Thus, according to the U.S. Pat. No. 3661567, the Mn-Al-C alloy magnets may be multi-component alloys having more then three components, containing impurities or additives other than Mn, Al and C, but should contain carbon as an indispensable component element, and moreover, the component ratio of Mn, Al and C in these multi-component alloys should fall within the following range:
Mn 69.5 .about. 73.0 % Al 26.4 .about. 29.5 % C 0.6 .about. (1/3Mn - 22.2) %
Then, only when these alloys are manufactured under the condition restricted as described in the following, the Mn-Al-C alloy magnets may be obtained as isotropic permanent magnets being in bulk form and excelling in their magnetic characteristics, stability, weathering resistance and mechanical strengths.
Thus, Mn, Al and C are so mixed that each component falls within the respective composition range mentioned above, then the mixture is heated to a temperature higher than 1,380.degree.C but lower than 1,500.degree.C, in order to obtain a homogeneous melt with carbon forcibly dissolved therein, and thereafter the molten alloy is cast in a suitable mold. The ingot thus-obtained is heated above 900.degree.C to form its high temperature phase, and then, is quenched by rapidly cooling it from a temperature above 900.degree.C to a temperature below 600.degree.C at a cooling rate of higher than 300.degree.C/min. The quenched alloy is then tempered by heating it at a temperature of 480.degree..about.650.degree.C for an appropriate period of time. A Mn-Al-C alloy magnet in bulk shape obtained in this way has magnetic characteristics better than (BH)max = 1.0 .times. 10.sup.6 G. Oe, while in an isotropic state. This magnetic characteristic runs twice as high as the magnetic characteristics of isotropic Mn-Al alloy magnets.
The present inventors have studied and analyzed the reasons why the magnetic characteristics of Mn-Al-C alloy magnets were improved expecially when the manufacturing conditions were restricted as described hereabove. As a result, it has been clarified that this improvement was due to the particular state of existence of carbon in the Mn-Al-C alloy magnets, i.e., the manufacturing conditions and their magnetic characteristics have an intimate relationship. Accordingly, under manufacturing conditions which make the state of existence of carbon inadequate, magnets having low magnetic characteristics can be produced which are in the same order as isotropic Mn-Al alloy magnets, even if the composition ratio of Mn, Al and C falls within the above mentioned ranges, and even wherein sufficient .tau. phase exists.
It was discovered that in order to obtain isotropic permanent magnets from Mn-Al-C alloys having excellent magnetic characteristics, it is necessary that the phases existing in these alloys should mainly include:
1. a magnetic phase having carbon forcibly melted therein beyond the solubility limit, and PA0 2. a phase of Mn.sub.3 AlC and/or a face-centered cubic phase resembling such phase in which the remaining excess carbon is separated out by way of tempering in the form of carbides other than aluminum carbide (Al.sub.4 C.sub.3, etc.) in fine grainy or reticular shape, and that phase (2) is separated and dispersed finely in grainy or reticular form within phase (1) as its matrix. It has been proven that when alloys are produced according to the above-described phase conditions, magnets having greatly improved magnetic characteristics can be manufactured, which alloys possess a stabilized magnetic phase. This state of existence of carbon, as described above, was confirmed by way of X-ray diffraction techniques, optical microscopy and electron microscopy. PA0 1. Excellent anisotropic magnets can be obtained by subjecting an alloy containing a composition of Mn 68.0.about.73.0%, C(1/10 Mn -- 6.6)%.about. (1/3 Mn -- 22.2)% and the remainder aluminum to plastic deformation at the temperature range of 530.about.830.degree.C after precipitating the phase of Mn.sub.3 AlC and/or face-centered cubic phase resembling such phase into plane (0001) of the high temperature phase. PA0 2. Excellent anisotropic magnets can be obtained by subjecting a polycrystalline alloy containing Mn (68.0.about.70.5)%, C(1/10 Mn -- 6.6)%.about.(1/3 Mn -- 22.2)% and the remainder aluminum, to a quench treatment at a cooling rate higher than 10.degree.C/min. in the temperature range of 830.degree..about.900.degree.C, then, tempering at the temperature of 480.degree..about.750.degree.C and subjecting to plastic deformation at the temperature of 600.degree..about.780.degree.C after the tempering.
Mn.sub.3 AlC is a compound having a face-centered cubic crystal structure of a perovskite type (lattice constant a = 3.87A), but because its Curie point is 15.degree.C, and it is nonmagnetic at room temperature, Mn.sub.3 AlC itself, even when existing in the Mn-Al-C alloys, does not contribute to the intensity of magnetization of the Mn-Al-C alloy magnets.
A face-centered cubic phase similar to Mn.sub.3 AlC means (1) a phase wherein perovskite type carbides appear in the Mn-Al-C alloys containing an amount of carbon beyond than the solubility limit, or (2) is a precipitated substance having the same chemical characteristics as that of said carbides, but not exactly the same structure.
Al.sub.4 C.sub.3 is a carbide existing in Mn-Al-C alloys containing Mn within the range of 68.0.about.73.0% and an amount of carbon in excess of (1/3 Mn--22.2)%. It is formed at temperatures above the melting points of Mn-Al-C alloys, but is neither formed nor destroyed by heat treatment in the temperature range below the melting points. Al.sub.4 C.sub.3, hydrolyzed by moisture in the air, etc., causes the alloys to crack, leading finally to the decay of alloys with the further proceeding of hydrolysis.
It has been clarified that in Mn-Al-C alloys, the solubility limit of carbon in the magnetic phase, as determined by the measurement of lattice constants by way of X-ray diffraction and by measurement of Curie point by use of a magnetic balance, is 0.7% for the composition of 73% Mn, 0.5% for the composition of 71% Mn, and the solubility limit of carbon within the composition range of 71.0.about.73.0% Mn can be represented by the mathematical formula of (1/10 Mn -- 6.6)%.
On the other hand, the solubility limit of carbon in the high temperature phase is almost the same as the solubility limit of carbon in the magnetic phase at a temperature of 830.degree.C, but in a temperature range of 900.degree.C.about.1200.degree.C, the solubility limit of carbon in this phase is (0.8.about.2.0)% of carbon; however, by quenching at a temperature above 900.degree.C, a high temperature phase can be obtained in which more than (1/10 Mn -- 6.6)% of carbon is forcibly dissolved. When, however, in the process of quenching from a temperature above 900.degree.C, a gradual cooling is made at a cooling rate lower than 10.degree.C/min. in the temperature range of 830.degree..about.900.degree.C, and then, quenching is carried out from this temperature, or when the alloys are held in the temperature range at 830.degree..about.900.degree.C for more than 7 minutes, preferably more than 10 minutes, and quenching from that temperature, Mn.sub.3 AlC precipitates in lamellae in a .epsilon. phase. The deposition of Mn.sub.3 AlC in lamella is oriented parallel to the special crystal plane of the .epsilon. phase, i.e., (0001) plane, and has the orientation relationship of EQU .epsilon.(0001) // Mn.sub.3 AlC (111)
as evidenced by optical microscopic observation and X-ray diffraction of a single crystal as the test specimen.
On the other hand, by subjecting the high temperature phase into which carbon has been forcibly dissolved to the aforementioned tempering at 480.degree..about.650.degree.C, a phase of Mn.sub.3 AlC and/or face-centered cubic phase resembling such phase is deposited, finely dispersed in the matrix of the magnetic phase into which free carbon more than the solubility limit has been forcibly contained.
The composition range of U.S. Pat. No. 3661567 refers to the most typical composition range, where the state of existence of carbon is concerned, and it was clarified that the excess carbon in the composition range especially of Mn more than 70% exists as the phase of Mn.sub.3 AlC and/or the face-centered cubic phase resembling such phase which has been deposited, finely dispersed in the grainy or reticular shape. The magnetic characteristic of the Mn-Al-C alloy obtained in this way is isotropic in the bulk state, with (BH)max running above 1.0 .times. 10.sup.6 G. Oe.
The present inventors invented anisotropic Mn-Al-C alloy magnets which are superior to the isotropic Mn-Al-C alloy magnets of U.S. Pat. No. 3,661,576. Inventions concerning anisotropic Mn-Al-C alloy magnets were disclosed that: